diff options
Diffstat (limited to 'clang/lib/Basic/Targets.cpp')
| -rw-r--r-- | clang/lib/Basic/Targets.cpp | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/clang/lib/Basic/Targets.cpp b/clang/lib/Basic/Targets.cpp index 17014873220..dbb7fb7b3fb 100644 --- a/clang/lib/Basic/Targets.cpp +++ b/clang/lib/Basic/Targets.cpp @@ -5802,6 +5802,27 @@ public: } const char *getClobbers() const override { + // In GCC, $1 is not widely used in generated code (it's used only in a few + // specific situations), so there is no real need for users to add it to + // the clobbers list if they want to use it in their inline assembly code. + // + // In LLVM, $1 is treated as a normal GPR and is always allocatable during + // code generation, so using it in inline assembly without adding it to the + // clobbers list can cause conflicts between the inline assembly code and + // the surrounding generated code. + // + // Another problem is that LLVM is allowed to choose $1 for inline assembly + // operands, which will conflict with the ".set at" assembler option (which + // we use only for inline assembly, in order to maintain compatibility with + // GCC) and will also conflict with the user's usage of $1. + // + // The easiest way to avoid these conflicts and keep $1 as an allocatable + // register for generated code is to automatically clobber $1 for all inline + // assembly code. + // + // FIXME: We should automatically clobber $1 only for inline assembly code + // which actually uses it. This would allow LLVM to use $1 for inline + // assembly operands if the user's assembly code doesn't use it. return "~{$1}"; } |
